It's slightly disconcerting to experienced electric drivers to feel the car's transmission up- or down-shift while in electric mode, but again, the shifts are so imperceptible that you have to pay close attention to feel it.

There are four basic driving modes: E, for conventional electric-then-hybrid operation, is the default. E+, the ultra-energy-saving mode, is not as annoying in this car as in many less expensive vehicles, and the performance remains tolerable--if not particularly lively.

The E-Save mode allows the driver to conserve battery charge for when it's needed, turning the car into a conventional hybrid even if the battery has capacity remaining.

And finally, there's Sport mode, which hustles the big sedan along noticeably more quickly if driven aggressively--but keeps the engine on throughout, with the electric motor providing boost under acceleration.

We noticed that using the paddle shifts behind the steering wheel to provide more immediate power also immediately switched on the engine.

Electric-only to 70 mph

Our test drive routes, which covered more than 60 miles all told, included highways, fast two-lane rural roads, and a fair amount of slower traffic through towns, villages, and traffic circles.

Under those circumstances, the plug-in S-Class stayed in electric mode an impressive amount of the time--although its maximum electric-only speed is 110 km/h (about 70 mph), far lower than average traffic on unlimited-speed autobahns.

Electric range appeared to be 15 to 18 miles, but we'll wait for EPA ratings, since the European test cycle tends to produce range ratings far higher--and less realistic--than those in North America.

Even at an indicated 150 km/h (93 mph), the car was smooth, quiet, and utterly confident--exactly the qualities that successful German businessmen buy them for.

Once in hybrid mode, the plug-in hybrid could have been a conventional S-Class for rear-seat passengers not paying much attention to the silent and smooth transportation.